Water softening process

ABSTRACT

This invention involves an improved process for softening hard water which comprises selectively precipitaing CaCO 3  to form a thin layer thereof, increasing the pH of said water to precipitate magnesium as magnesium hydroxide and then filtering the resultant slurry through said layer. The CaCO 3  layer serves as a thin permeable layer which has particularly useful application in cross-flow filtration applications.

BACKGROUND OF THE INVENTION

The invention described herein was made in the course of, or under, acontract with the United States Atomic Energy Commission.

The present invention relates to an improved method of softening waterhardness caused by CaCO₃ and Mg(OH)₂. More particularly, it relates to aprocess for treating effluents of filtrates resulting from the lime-sodamethod to low levels of total hardness of the order 10 ppm as CaCO₃ orless.

In the lime-soda method for softening water, slaked lime Ca(OH)₂, isadded to hard water to convert water soluble calcium and magnesiumbicarbonate to water insoluble calcium carbonate and magnesiumhydroxide. Resulting filtrates contain levels of hardness which preventefficient further treatment of the water by hyperfiltration processes toreduce total salinity, for example, as described in U.S. Pat. No.3,743,595. Studies have shown that desalination of hard brackish watersby hyperfiltration with dynamic membranes does not proceed efficientlyand for these hyperfiltration processes the hardness of feed watersshould not exceed more than 10 ppm total hardness. (The degree of totalhardness in water, no matter to what compounds it may be due, isexpressed in terms of calcium carbonate, CaCO₃.)

A particularly convenient way of conducting filtration of lime-sodafiltrates to produce a product water having the required low level oftotal hardness is achievable by the process of crossflow filtration.However, a technically and economically competitive cross-flowfiltration process must provide a filtration flux of at least 200 andpreferably 300 gallons per square foot of filtration surface area perday, gal/ft² -day.

It is, accordingly, a principal object of this invention to provide aprocess which produces a product water suitable for further treatment byhyperfiltration or for any other application requiring a low hardnessfeed. More particularly, it is an object of this invention to provide afiltration process capable of providing a water product containing nomore than 10 ppm total hardness at a production level of at least 200gallons per square foot of filtration surface area per day. A furtherobject is to provide an improved cross-flow filtration process capableof realizing the aforementioned objects. Attainment of these and otherobjects will become apparent from the ensuing description in which theaccompanying FIGURE is a graph which shows the relationship betweenproduct flux and pH in the course of a cross-flow filtration test andthe relationship of these two parameters as a function of run time.

SUMMARY OF THE INVENTION

The present invention is predicated on the discovery that in the firstinstance that a precipitated layer of CaCO₃ serves as an effectivepermeable filtration layer through which aqueous feeds can be passed toproduce satisfactorily low levels of hardness at suitably high productfluxes. The gist of the invention involves a pH-controlled selectiveprecipitation of CaCO₃ from a hard water feed sufficient to form apermeable filtration layer on a porous substrate, and then raising thepH to precipitate remaining calcium values as well as Mg(OH)₂. Selectiveprecipitation of CaCO₃ and then passing the pH-adjusted feedtangentially across said filter layer of CaCO₃ at a sufficient pressureand velocity results in a low hardness, clarified filtrate collected onan opposite face of said porous substrate which can be processed byhyperfiltration techniques to effect desalting.

In cross-flow filtration, a liquid feed is flowed tangentially past afiltering surface. Operationally, it involves pumping feed through aporous pipe or tube support substrate upon which is deposited a thinpermeable layer of finer porosity than that of the support substrate.Feed water passes across the permeable deposited layer and across thetube wall to be collected as a filtered product on the opposite side ofthe tube. A fairly wide variety of porous tubular substrate elements maybe used including porous ceramic, carbon, metal, or plaster tubes orscreens. A particularly convenient class of materials is woven tubularfabrics made similar to fire hose jackets. Such flexible forms of filtergenerally require support when pressurized feed is applied from theoutside and product is collected within the tube. Reinforcement is notrequired where feed is pumped through the tube and product collected asit forms across a thin permeable filter layer deposited in the internalsurface of the tube and thence across the tube wall. The thin permeablelayer of CaCO₃ used in the present invention may be predeposited on thefeed side or deposited dynamically from the feed solution.

According to the present invention, an effective filtration membrane isdynamically formed by selective precipitation of CaCO₃ from a hard waterfeed by adjusting or maintaining the feed at a pH in the range 8 to 10at a temperature in the range 15° to 35°C. Under these conditions apermeable filtration layer consisting essentially of CaCo₃ isselectively precipitated and deposited on the surface of the selectedtubular substrate. To remove the remaining total hardness, the feed pHis then adjusted to the range 10.8 to 11.5 to precipitate additionalCaCO₃ and the Mg as Mg(OH)₂ to produce a water product containing lessthan 10 ppm total hardness. Satisfactory product fluxes are obtained ata cross-flow velocity ranging from 3 to 35 ft/sec at feed pressures inthe range 15 to 70 psig. The velocity should not be as low as to allowbuild-up of an excess amount of CaCO₃ but not so high as to shear offthe effective filtration layer of CaCO₃, generally thought to amount tono more than a few microns in thickness.

The following examples illustrate representative embodiments of how touse the invention and provide tangible evidence of the advantagesresulting therefrom.

The process of the present invention is a modified form of theclassically well-known lime-soda process for softening hard watercoupled with cross-flow filtration. A principal feed source for thepresent invention is derived from the conventional lime-soda processwhere hardness on a large scale is removed by addition of slaked lime inlarge tanks. Precipitated hardness is allowed to settle. The resultantoverlaying dilute solution or slurry or filtrate can serve as feed forthe improved cross-flow filtration process of the invention. Feedhardness as high as 5000 ppm has been processed by the presentinvention.

EXAMPLE I

A synthetically prepared hard brackish water feed was prepared to thefollowing composition in mg/l: Na, 465; Ca, 160; Mg 51; Cl, 710; SO₄ ⁼,391; HCO₃ ⁻, 229; and Fe, .02, total hardness 610. The feed was raisedto a pH of about 12, by addition of NaOH to convert the bicarbonate tocarbonate for precipitation of part of the feed calcium and providehydroxyl ion for magnesium precipitation. Then CO₂ was added to completeprecipitation of Ca. The reactions involved may be represented asfollows:

    OH.sup.- + HCO.sub.3 → CO.sub.3 .sup.- .sup.- + H.sub.2 O

    2oh.sup.- + co.sub.2 → co.sub.3 .sup.- .sup.- + h.sub.2 o

    ca.sup.+.sup.+ + CO.sub.3 .sup.- .sup.- → CaCO.sub.3 ↓

    mg.sup.+.sup.+ + 2OH.sup.- → Mg(OH).sub.2 ↓

the slurry from the thus treated solution was filtered in a cross-flowfiltration loop consisting of a length of nominal 1-inch diameter nylonpolyester fire hose jacket having a filtration area of 0.83 ft². Feedwas circulated through the hose and a water product collected as thewater transpired through the jacket. The table below summarizes theresults of this treatment as run 1 with four subsequent runs repeatedunder defined conditions.

                                      TABLE                                       __________________________________________________________________________    Cross-Flow Filtration of Calcium and Magnesium Precipitates                   From Synthetic Hard Water Feed (˜30°C., Initial Feed Hard-       ness: ˜720 ppm Total Hardness as CaCO.sub.3, 480 ppm Calcium            as CaCO.sub.3)                                                                __________________________________________________________________________                      Product                                                        Run  Feed Inlet     Total                                                  Run                                                                              Time Velocity                                                                           Pressure                                                                           Flux Hardness                                               No.                                                                              (hr) (ft/sec)                                                                           (psig)                                                                             (gpd)                                                                              (ppm CaCO.sub.3)                                                                      pH                                             __________________________________________________________________________    1  0.3  4.1  26   89    9      10.8                                              19   4.1  26   69   16      11.7                                           2  1.2  5.8  51   121   9      11.4                                              5.7  5.8  51   114   9      11.4                                              20.5 5.8  51   96   100     10.3.sup.a                                     3  1.0  6    54   90    4      11.5                                              6.5  6    54   89           11.1                                           4  1.2  3.4  15   2070.sup.b                                                                         246.sup.b                                                                             9.9.sup.b                                         20.5 3.4  15   1130 346     8.4                                            5  0.6  6.7  70   813  12      11                                                7.6  6.7  70   835   9      10.8                                              22.8 6.7  70   350  48      10.3.sup.a                                     __________________________________________________________________________     .sup.a pH decrease during overnight operation.                                .sup.b Treatment for calcium removal, only 8 ppm Ca.sup.+.sup.+ in            product.                                                                 

In the first run, with a modest cross-flow velocity of only 4 ft/sec,the product flux averaged 79 gallons per day over 19 hours. Ofparticular significance is the low-level average product hardness ofonly 9-16 ppm as compared to 30-40 ppm hardness obtained by conventionalwater softening techniques employing sand filtration or sedimentation,or by hot lime-soda operation. Run 2 was operated at slightly highervelocity and nearly doubled pressure; flux was somewhat higher andproduct hardness was 9 ppm for several hours increasing only with adecrease in pH. Run 3 conditions were similar to those of run 2 withproduct hardness as low as 4 ppm. Runs 4 and 5 illustrate the invention.In run 4, the pH of the feed was held between 8 and 10 to solubilize Mgand precipitate calcium only. Here cross-flow filtration at a velocityof 3.4 ft/sec and 15 psig inlet pressure resulted in a dramatic increasein product flux; the total hardness of 246-346 ppm reflected primarilythe magnesium contribution to the total hardness and included only 8 ppmCa⁺ ⁺. The final (5th) run illustrates the best mode of operating theinvention. In this case, the pH was raised high enough (10.8 to 11) toprecipitate magnesium. When this feed was circulated through the hosecontaining a precoated layer of CaCO₃ from previous run 4, an averageproduct flux of over 500 gpd/ft² was attained at satisfactorily lowhardness levels. After the start up transient during which producthardness decreased to less than 10 ppm, product hardness remained atthis low level for several hours until decreased feed pH resulted insolubilizing some magnesium.

Viewing the overall results and comparing the runs, it is seen thatselective precipitation of CaCO₃, as in run 4, apparently develops acondition of permeability sufficient to allow attainment of high productfluxes; subsequent precipitation of Mg(OH)₂ does not adversely affectfiltration product flux. In cases where calcium and magnesium weresimultaneously precipitated (as in runs 1-3), product flux wasunsatisfactorily low although the product hardness level was generallyacceptable.

The low product fluxes are attributable to the relatively nonpermeablegelationous Mg(OH)₂. Yet, surprisingly, the Mg(OH)₂ does not appear tointerfere with obtaining satisfactorily high product fluxes when aprecoat of CaCO₃ is laid down.

The brackish water products from the runs shown in the table were usedas feed to test their behavior as feed for treatment by hyperfiltrationin accordance with general procedures described in U.S. Pat. No.3,743,595. Specifically, the effect of the different treatments in theperformance of a Zr IV-polyacrylic acid dual layer hyperfiltrationmembrane was studied. After formation of the dual layer a base line ofperformance with 0.058M NaCl with no calcium or magnesium wasestablished during 140 hours of operation at 1000 psig. Chloriderejection was 92% at a flux of 90 gpd/ft². When feed was switched topretreated brackish water with 8 ppm total hardness (run 4) the productflux was essentially unchanged, but the chloride rejection decreasedfrom 92% to 80% and sulfate rejection increased slightly from 94% to96%. Increasing the hardness level in the pretreated feed to 100 ppmresulted in increases in both chloride (80 to 88%) and sulfate (90 to98%) rejection. However, the flux decreased from 90 to 30 gpd/ft². Useof an untreated synthetic hard water feed (970 ppm total hardness)resulted in decreases in chloride (˜70%) as well as sulfate (82%)rejection and a sharp decrease in flux to about 17 gpd/ft². A summary ofresults shows that maintenance of favorable hyperfiltration performanceboth in terms of flux and ion rejection requires feed with no more than10 ppm total hardness.

EXAMPLE II

In this example cross-flow filtration studies were made using an 18-footlong section of 1-inch diameter fire hose jacket having a filter surfacearea of about 4.5 ft². The hose was pretreated by placing in a dilutehydrochloric acid solution (pH 2-5) followed by demineralized water inorder to dissolve and flush out residual CaCO₃ and Mg(OH)₂ from previousruns. A brackish water feed (1130 ppm total hardness, 840 ppm calciumhardness) pressurized to 35 psig was circulated through the fire hosejacket at 35°C at a velocity of 16 ft/sec at an initial feed pH of ˜8.4for a period of 0.5 hour to deposit precipitated CaCO₃. The pH of thefeed was then raised to 11.3 for the remainder of the filtration run.The FIGURE shows a product flux, feed pH profile as a function of time.Of the 840 ppm initial calcium hardness no more than 100 ppm was insolution at pH 8.4, the remainder having been precipitated and depositedas a smooth layer of CaCO₃ on the internal surface of the fire hosejacket. After 0.5 hour the pH was increased over a 1.5 hour period to afinal pH of 11.4. At pH 10, the flux was still as high as 2700 gal/ft²/day, but decreased steadily until it stabilized at 600 gal/ft² /day. Atabout 2 hours, after final feed adjustment, the total product hardnesswas less than 5 ppm.

It will thus be seen that we have shown how a precipitated layer ofCaCO₃ can serve as an effective filtration layer for use in cross-flowfiltration applications. For achieving effective filtration ofhard-water feeds in terms of desirably low product hardness and maximumproduct flux, the Ca is selectively precipitated while maintainingmagnesium in solution. The novel use of CaCO₃ has the added virtue thatit can be easily removed simply by flushing with an acid solution. Thus,in cases where excess buildup of CaCO₃ reduces flow and/or flux, atemporary flow of acidified solution will enable rejuvenation ofsubsequent deposition of a fresh CaCO₃ filtration layer in the mannerhereinbefore described.

What is claimed is:
 1. An improved process for softening hard water feedwhich comprises selectively precipitating CaCO₃ from said feed at a pHsufficient to deposit a permeable filtration layer consistingessentially of CaCO₃ on a porous substrate, while flowing said feedtangentially past said substrate increasing the pH of said feed toprecipitate magnesium as magnesium hydroxide and then flowing the thustreated water tangentially past the deposited CaCO₃ at a velocity andpressure sufficient to effect transpiration of the water through saidCaCO₃ and substrate to produce a transpired clarified water product. 2.An improved process for softening hard water feed which comprisesadjusting said feed to a pH in the range 8 to 10 and a temperature inthe range of 15° to 35°C while flowing said water tangentially past asurface of a tubular porous substrate to cause deposition of a permeablefiltration layer consisting essentially of CaCO₃ on said surface,adjusting said feed to a pH in the range of 10.8 to 11.5 which effectsprecipitation of magnesium as Mg(OH)₂ and then flowing the thus treatedwater tangentially past said CaCO₃ layer at a velocity and pressuresufficient to collect a transpired clarified water product from anopposite surface of said substrate.
 3. The method according to claim 2in which the substrate is a woven fabric.
 4. The method according toclaim 2 in which the velocity of the feed water flowing past saidpermeable layer of CaCO₃ is from 3 to 35 ft/sec and is at a pressure offrom 15 to 70 psig.